1. Technical Field
The present invention generally relates to polymer foam sheet formation. More particularly, the present invention relates to polymer foam sheet formation using only ambient gas blowing agents.
2. Background Information
Polymer products produced by thermoforming of polymer sheet (e.g., polystyrene sheet) have proven useful in a great many applications. One of the major applications for this type of product is disposable food packaging. Historically, there has been some marketplace resistance to this product due to environmental concerns. One of the first major issues was concern of the destruction of the ozone layer due to the release of the Chloroflourocarbons used as blowing agents. Chloroflourocarbons were eliminated and mostly replaced with hydrocarbons or hydrocarbon/carbon dioxide blends. Concern over landfills then became the next major environmental issue addressed and has been somewhat alleviated through ongoing recycling programs. A current concern is, however, the use of hydrocarbon blowing agents. These gasses escape from the product over time, causing concern about ozone generation at ground level. Hydrocarbon blowing agents also present a practical problem to producers, as cumbersome environmental permits are required, as well as expensive pollution abatement equipment. Pollution abatement, however, addresses only those hydrocarbons released at the plant location and does not address the hydrocarbon gasses that are shipped out of the production plant as part of the product and released later. In addition, these gasses are highly flammable, resulting in expensive equipment and sensors designed for flammable environments, high insurance premiums, and most important, occasional worker injuries caused by fire.
In an effort to minimize the problems noted above with the use of hydrocarbon blowing agents, many companies have resorted to using hydrocarbon blends with ambient gasses, most significantly carbon dioxide. However, commercial technology generally limits the amount of carbon dioxide that can be used to less than 50 mole percent of the blowing agent present at manufacture. Carbon dioxide has a very high vapor pressure compared with the hydrocarbon blowing agents. It therefore causes rapid expansion of the foam mass on exit of the die which results in sheet corrugation, thin gauge, irregular cell size, and/or poor surface appearance. Corrugation is defined as numerous gauge bands in the machine direction of foam sheet causing local gauge variations of greater than 5%. Limiting the problems associated with the use of carbon dioxide limits the amount that can be successfully used in manufacture without compromising quality or increasing product weight. Any attempts to use only carbon dioxide as a blowing agent have resulted in product that is of either inferior quality or more than 10% greater in weight compared to product made using at least a portion of hydrocarbon blowing agents. These problems are magnified for other ambient gasses, as they have vapor pressures much higher than carbon dioxide.
An additional problem found with the use of ambient gasses is the lack of post expansion when thermoforming the sheet. The permeation rate of hydrocarbons through polystyrene is much slower than that of ambient gasses. As a result, upon aging of the sheet for a number of days, the partial pressure inside the foam sheet is significantly greater than one atmosphere. Thus, by the ideal gas law, the sheet can be expected to expand between 50% and 120% when heated to the glass transition temperature. Without this increase in partial pressure, only thermal expansion effects of 10% to 40% gauge increases are realized. This expansion is necessary to produce the low densities required of the final product. It is known that the strength-to-weight ratio improves for foam as the density is reduced. Therefore, without this expansion, the product would be weaker and require more weight to compensate, which again has a severe negative economic impact. One potential solution is to reduce the density of the sheet such that further density reduction requirements at forming are not needed. This solution requires more blowing agent, which exasperates all of the negatives associated with these ambient blowing agents already described.
There exists, therefore, a need to produce polymer foam sheet using only ambient gasses that has little or no corrugation, uniform cell structure, and low density. Such sheet could then be thermoformed into the final product desired without either reduction in quality or increase in product weight.
Briefly, the present invention satisfies the need to produce polymer foam using only one or more ambient gasses as blowing agents by providing an annular die designed to address the problems resulting from their exclusive use.
In accordance with the above, it is an object of the present invention to provide an annular die for producing polymer foam using only one or more ambient gasses as blowing agents.
The present invention provides, in a first aspect, a method of producing polymer foam. The method comprises heating a polymer resin to its melt temperature, selecting at least one blowing agent consisting of at least one ambient gas, combining the heated polymer resin with the blowing agent(s) to create a mixture, and extruding polymer foam sheet from the mixture comparable in quality to that obtainable with hydrocarbon blowing agents.
The present invention provides, in a second aspect, an annular die for producing polymer foam. The annular die comprises an exiting channel having an exit with a cross-sectional area larger than at least one point within the exiting channel.
These, and other objects, features and advantages of this invention will become apparent from the following detailed description of the various aspects of the invention taken in conjunction with the accompanying drawings.